1. Field of the Invention
The present invention relates to a pressure sensor-incorporating spark plug which, is attached to an internal combustion engine in order to generate spark discharge within a cylinder and includes a piezoelectric element for detecting the internal pressure of the cylinder.
2. Description of the Related Art
A spark plug is attached to an internal combustion engine in order to generate spark discharge within a cylinder and thereby burn an air-fuel mixture. In relation to such a spark plug, a pressure sensor-incorporating spark plug is known which includes a built-in pressure sensor for detecting the internal pressure of the cylinder and for outputting an electric signal corresponding to the detected pressure.
FIG. 6 shows an exemplary conventional pressure sensor-incorporating spark plug. The pressure sensor-incorporating spark plug 101 shown in FIG. 6(a) includes a substantially cylindrical metallic shell 17 to be attached to an internal combustion engine; a substantially cylindrical insulator 15, which is held by the metallic shell 17 such that its center axis coincides with the center axis of the metallic shell 17; an electricity reception terminal 13, which is disposed at one axial end of the insulator 15 and which receives high voltage for ignition; a center electrode 19, which is disposed at the other axial end of the insulator 15 and is electrically connected to the electricity reception terminal 13 via the interior of the insulator 15; and a ground electrode 21, which is provided at an axial end of the metallic shell 17 corresponding to the axial end of the insulator 15 at which the center electrode 19 is provided.
A radially bulged flange portion 17b is formed at a longitudinal center portion of the metallic shell 17. The flange portion 17b includes a hexagonal portion 17a, which is formed at a portion closer to the electricity reception terminal 13 and onto which a tightening tool is fit; and a sensor case 29, which accommodates a piezoelectric element (not shown) for outputting an electric signal corresponding to the internal pressure of a cylinder. A bottom portion of the sensor case 29 serves as a mounting seat 17g, which abuts a cylinder head upon attachment of the spark plug to the internal combustion engine.
The pressure sensor-incorporating spark plug 101 further includes an output cable 27 for leading to external equipment the electric signal output from the piezoelectric element provided on the metallic shell 17. The output cable 27 is passed through a metal pipe 35, which is fixed to a side surface of the hexagonal portion 17a, and is fixed to the metal pipe 35 to thereby fix the output cable 27 to the hexagonal portion 17a. Thus, the output cable 27 extends from the pressure sensor-incorporating spark plug 101.
The pressure sensor-incorporating spark plug 101 is attached to the cylinder head of the internal combustion engine by means of a screw-engagement groove 17c, which is formed on the metallic shell 17 located between the flange portion 17b and the ground electrode 21. However, as shown in FIG. 6(b), which is a view from above the pressure sensor-incorporating spark plug 101 (as viewed in the direction of arrow B in FIG. 6(a)), the metal pipe 35 is fixed to the side surface of the hexagonal portion 17a, making it impossible to fit an ordinary plug wrench onto the hexagonal portion 17a. Therefore, when the pressure sensor-incorporating spark plug 101 is attached to the cylinder head of the internal combustion engine, a dedicated plug wrench having a slit at a portion corresponding to the metal pipe 35 must be used.
In addition, if such a plug wrench having a slit is used to tighten or remove the pressure sensor-incorporating spark plug 101 in a state in which the plug wrench has not been completely fit onto the hexagonal portion 17a and thus the output cable 27 engages a slit wall, the output cable 27 may be damaged, and in some cases may be pulled out.
In view of this problem, a pressure sensor-incorporating spark plug disclosed in Japanese Utility-Model Application Laid-Open (kokai) No. 60-138292 has been proposed. This pressure sensor-incorporating spark plug is configured such that an output cable is passed through a through hole formed in the hexagonal portion. That is, since the output cable is led outward from an upper end surface (end surface on the electricity reception terminal side), the hexagonal portion can maintain a hexagonal cross section to which an ordinary plug wrench can be fit. Thus, the pressure-sensor-incorporating spark plug can be tightened or removed using an ordinary plug wrench.
The above-described structure eliminates an accident in which the output cable engages a slit wall of a plug wrench, to thereby prevent damage to or pulling out of the output cable.
3. Problems to be Solved by the Invention
However, the pressure sensor-incorporating spark plug described in the above patent publication (Japanese Utility-Model Application Laid-Open No. 60-138292) has a drawback, in that the output cable is susceptible to being pulled out. This is because the output cable is fixed to the wall of the through hole of the hexagonal portion by means of a heat-resistant resin, which is charged into the space between the output cable and the wall of the through hole.
In view of this drawback, the above patent publication discloses an embodiment in which the wall of the through hole is roughened in order to improve the output cable""s resistance to being pulled out. However, this raises a problem of an increase in the number of work steps for fixing the output cable.
In view of the forgoing problems of the prior art, an object of the present invention is to provide a pressure sensor-incorporating spark plug which has a tightening portion onto which an ordinary plug wrench having no slit can be fit, which has an output cable having increased resistance to being pulling out, and which facilitates work for fixing the output cable.
In order to achieve the above object, a first aspect of the invention provides a pressure sensor-incorporating spark plug comprising: a spark plug main body including a substantially cylindrical metallic shell having a radially bulged flange portion at a center portion of the metallic shell, the flange portion having a mounting seat which abuts a cylinder head upon attachment of the spark plug to an internal combustion engine, and a tightening portion which is provided at a location apart from the mounting seat and onto which a tightening tool is fit, the tightening portion having a cross-sectional shape having a vertex portion; a piezoelectric element attached to the metallic shell and outputting an electric signal corresponding to cylinder internal pressure of the internal combustion engine; and an output cable for leading the electric signal generated by the piezoelectric element to the outside. The pressure sensor-incorporating spark plug is further characterized in that a cable accommodation portion for accommodating the output cable is provided at the vertex portion of the tightening portion, and the output cable is placed in the cable accommodation portion and is fixed to the tightening portion by crimping a portion of the tightening portion around the cable accommodation portion.
That is, in the pressure-sensor-incorporating spark plug of the first aspect of the invention, the output cable is placed in the cable accommodation portion and is fixed to the tightening portion by crimping a portion of the tightening portion around the cable accommodation portion, so that the tightening portion nips the output cable to thereby fix the output cable. The cable accommodation portion may be a through hole, where the output cable is passed through the through hole.
Since a structure for fixing the output cable by crimping is employed, the number of work steps can be decreased as compared with the case in which the output cable is fixed to the tightening portion by charging of, for example, a heat resistant resin. Thus, the work for fixing the output cable can be simplified. Further, since the output cable is fixed by crimping, the strength for withstanding pull out (pulling-out resistance) increases, the pull out occurring when an external force is applied to the output cable in the direction for pulling out the output cable.
The cross-sectional shape of the tightening portion having a vertex portion must be determined such that the shape prevents slippage of a tightening tool (e.g., plug wrench), which is fit onto the tightening portion in order to rotate the spark plug about the center axis. An example of such a shape is a polygon such as a hexagon or a pentagon; and the vertex portion refers to a portion in the vicinity of a vertex of the polygon.
With regard to the distance between a side of the polygon and the insulator as measured on the end surface of the tightening portion on the side of the electricity reception terminal, the distance between a vertex of the polygon and the insulator is greater than the distance between a central portion of a side of the polygon and the insulator. Therefore, at a vertex of the polygon, a larger area can be used for providing the cable accommodation portion. In the pressure sensor-incorporating spark plug of the first aspect of the invention, since the cable accommodation portion is provided at a vertex portion of the polygonal portion, a wide region can be secured for disposing the output cable. Accordingly, by virtue of the structure of the cable accommodation portion being provided at a vertex portion of the tightening portion, the output cable does not project from the side surface of the tightening portion. Therefore, the tightening portion can maintain a cross section which enables an ordinary plug wrench to be fit onto the tightening portion.
Therefore, in the first aspect of the invention, since the output cable is fixed by crimping, the number of work steps can be decreased, whereby the work for fixing the output cable can be simplified. Further, a sufficiently high pulling-out resistance can be secured. Moreover, since the cable accommodation portion is provided at a vertex portion of the tightening portion, the tightening portion can maintain a cross section which enables an ordinary plug wrench to be fit onto the tightening portion. Since an ordinary plug wrench can be fit onto the tightening portion, use of a plug wrench having a slit becomes unnecessary, and accidental pulling out of the output cable is prevented.
The cable accommodation portion may be a through hole which penetrates the interior of the tightening portion from an end surface on the side of the electricity reception terminal to an end surface on the side of the mounting seat. Alternatively, the cable accommodation portion may be a groove formed on a side surface of the tightening portion, which surface comes into contact with the inner surface of the plug wrench. As such, the groove extends from the end surface on the side of the electricity reception terminal to the end surface on the side of the mounting seat.
Typically, the output cable consists of a core wire made of an electrically conductive material and a covering layer made of an insulating material and covering the periphery of the core wire. An electric signal generated at the piezoelectric element is output to the outside via the core wire. Further, in one known type of such an output cable, the periphery of the output cable is covered with a shield material made of an electrically conductive material, in order to suppress influence of external noise on an electric signal flowing through the core wire. Such a shield material can prevent superposition of external noise on the electric signal flowing through the core wire disposed inside the cable, because the shield material is made of an electrically conductive material, and is electrically connected to ground (earth). Therefore, the shield material must be electrically connected to ground in a reliable manner.
The shield material may be connected to ground by providing wiring in order to ground the shield material via a connector provided at the end portion of the output cable opposite the piezoelectric element. However, when the shield material is grounded by means of such a method, the shape of the connector becomes complicated. Also, work for providing a wiring line from the connector to the ground becomes necessary, thereby increasing the number of work steps.
The metallic shell of the pressure sensor-incorporating spark plug is made of an electrically conductive material in order to form a path for applying high voltage for ignition, and is grounded upon attachment of the spark plug to an internal combustion engine. Therefore, when the shield material is brought into contact with the tightening portion of the metallic shell, the shield material can be electrically connected to ground.
In view of the foregoing, when an output cable whose periphery is covered with a shield material for suppressing influence of external noise is used in the above-described pressure sensor-incorporating spark plug of the first aspect, as described in a second aspect of the invention, the shield material is preferably grounded via the tightening portion, by establishing contact between the shield material and the tightening portion.
That is, when the tightening portion is deformed by crimping, the output cable is fixed to the tightening portion, and simultaneously, the shield material is brought into contact with the tightening portion to thereby establish electrical connection between the shield material and the tightening portion. Therefore, when the pressure sensor-incorporating spark plug is attached to an internal combustion engine, the metallic shell is electrically connected to the ground, so that the shield material is connected to ground.
Therefore, in the second aspect of the invention, by crimping work, wiring for grounding the shield material can be formed simultaneously with fixing of the output cable. Thus, separate work for providing wiring for grounding the shield material becomes unnecessary, so that the number of steps can be reduced.
Incidentally, when the cable accommodation portion is formed by a through hole, the operation of nipping the output cable by deforming the tightening portion by crimping can be performed with relative ease when the cross-sectional area of the through hole is set to a size corresponding to the cross-sectional area of the output cable.
However, when the cable accommodation portion is formed by a groove, a portion which deforms due to crimping cannot attain a sufficiently large volume by merely forming a groove on the tightening portion. In view of the foregoing, in order to reliably nip the output cable accommodated in the cable accommodation portion in the form of a groove, desirably an extension portion is provided which extends from the tightening portion at the vicinity of the cable accommodation portion, and the extension portion is deformed by crimping to thereby nip the output cable. However, when such an extension portion is provided, the shape of the tightening portion becomes complex, and the number of work steps for forming the tightening portion increases.
A third aspect of the invention, which has been accomplished to solve the above-described problem, provides a pressure sensor-incorporating spark plug comprising: a spark plug main body including a substantially cylindrical metallic shell having a radially bulged flange portion at a center portion of the metallic shell, the flange portion having a mounting seat which abuts a cylinder head upon attachment of the spark plug to an internal combustion engine, and a tightening portion provided at a location apart from the mounting seat and onto which a tightening tool is fit, the tightening portion having a cross-sectional shape having a vertex portion; a piezoelectric element attached to the metallic shell and outputting an electric signal corresponding to cylinder internal pressure of the internal combustion engine; and an output cable for leading to the outside the electric signal generated by the piezoelectric element. The pressure sensor-incorporating spark plug further comprise a groove for accommodating the output cable provided on a surface of the tightening portion, and a fixing member for fixing the output cable to the groove, wherein the output cable is fixed to the groove by means of the fixing member.
That is, in the pressure sensor-incorporating spark plug of the third aspect of the invention, the output cable is placed in the groove and is fixed to the tightening portion by means of the fixing member.
By virtue of the structure for fixing the output cable by means of the fixing member, the work for forming on the tightening portion an extension portion or a like for nipping the output cable can be eliminated. Further, the output cable can be reliably fixed to the groove.
In an example case in which the tightening portion is formed to have a hexagonal cross section, the groove is provided at a vertex portion of the hexagon. In such case, if an extension portion is provided at the vicinity of the groove and the tightening portion (extension portion) is deformed by crimping in order to nip the output cable, the extension portion must be formed to have a considerably large size. In such case, the work for forming the tightening portion becomes more complicated. By contrast, in the third aspect of the invention, the fixing member enables easy fixing of the output cable without providing an extension portion.
In a fourth aspect of the invention, the fixing member is preferably a member having a shape for surrounding the output cable. After the output cable is passed through the fixing member, the fixing member is placed in the groove. In such state, the fixing member is deformed in a direction perpendicular to the passing direction of the output cable. As a result, the fixing member expands along the width direction of the groove to thereby be held by the groove, and shrinks along the depth direction of the groove to thereby nip the output cable.
An example of the fixing member having a shape for surrounding the output cable is a fixing member having a tubular shape.
After the fixing member, through which the output cable has been passed, is placed in the groove, the fixing member is deformed such that the fixing member expands in the width direction of the groove. As a result, the outer surface of the fixing member comes into contact with the wall surface of the groove, and, by virtue of stress generated by deforming the fixing member, the fixing member is held by the wall surfaces (opposite side surfaces with respect to the width direction) of the groove.
Further, by virtue of such deformation of the fixing member, the fixing member shrinks along the depth direction of the groove. Therefore, the fixing member nips the output cable in a state in which the inner surface of the fixing member is in contact with the surface of the output cable.
As described above, the output cable is nipped by the fixing member; and the fixing member is fixedly held within the groove of the tightening portion. As a result, the output cable is fixed to the tightening portion.
Therefore, in the fourth aspect of the invention, by using the fixing member, the output cable can be reliably fixed to the groove formed on the tightening portion.
Notably, a seamless pipe-like member may be used as the fixing member having a shape for surrounding the periphery of the output cable. Alternatively, a plate member bent into a tubular shape may be used as the fixing member.
Further, a fixing member which does not surround the periphery of the output cable may be used. An example of such a fixing member is a wedge-shaped fixing member, which is inserted into the clearance between the output cable and the wall surface of the groove to thereby fix the output cable to the groove.
When the periphery of the output cable is covered with a shield material for suppressing influence of external noise, as described previously, wiring for connecting the shield material to the ground must be provided, thereby increasing the number of work steps.
In view of the foregoing, when an output cable whose periphery is covered with a shield material for suppressing influence of external noise is used in the above-described pressure-sensor-incorporating spark plug (the third and fourth aspects of the invention), in a fifth aspect of the invention, the fixing member is preferably formed of an electrically conductive material, and the shield material is preferably connected electrically to the tightening portion via the fixing member, whereby the shield material is grounded via the tightening portion.
That is, when the output cable is fixed to the groove by using the fixing member, the shield material comes into contact with the fixing member, and the fixing member comes into contact with the tightening portion, whereby electrical connection is established between the shield material and the tightening portion via the fixing member. The fixing member fixed to the groove is electrically connected to the metallic shell; and when the pressure sensor-incorporating spark plug is attached to an internal combustion engine, the metallic shell is electrically connected to the ground. Thus, the shield material is connected to the ground.
Therefore, in the fifth aspect of the invention, simultaneously with fixing of the output cable by means of the fixing member, wiring for grounding the shield material can be formed. Thus, separate work for providing wiring for grounding the shield material becomes unnecessary, so that the number of steps can be reduced.
When a member, such as a tubular member, which surrounds the radial periphery of the output cable is used as a fixing member, the surface of the output cable (specifically, the shield material) does not come into direct contact with the wall surface of the groove. Therefore, in such a case, the fixing member must be made of an electrically conductive material. However, when a fixing member, such as a wedge-shaped fixing member, which can establish direct contact between the surface of the output cable (specifically, the shield material) and the wall surface of the groove is employed, the fixing member may be made of a material having no electrically conductivity.
Incidentally, when a connector is attached to an end portion of the output cable, an electrode terminal disposed inside the connector is connected to the core wire of the output cable by means of crimping. In such a case, as compared with an output cable having a thin core wire, an output cable having a thick core wire enables a crimping portion of the electrode terminal to hold the core wire more reliably, to thereby lower the possibility of the electrode terminal coming off. Further, as compared with an output cable having a thin core wire, an output cable having a thick core wire has a lower line resistance, to thereby reduce power loss in the line.
However, in order to use an output cable having a thick core wire, a region (area) which enables such output cable to be disposed must be secured on the end surface of the tightening portion on the side of the electricity reception terminal.
In view of the foregoing, when an output cable having a thick core wire is used in the above-described pressure sensor-incorporating spark plug (any of the first through fifth aspects of the invention), as described in a sixth aspect of the invention, the tightening portion preferably has a cross section of a hexagon; the distance between opposite sides of the hexagon is preferably 15.5 to 16 mm; and the difference between the diameter of the insulator and the distance between opposite sides of the hexagon is preferably 6.3 to 7.2 mm.
In most conventional pressure sensor-incorporating spark plugs, in general, the tightening portion has a hexagonal cross section; the distance between opposite sides of the hexagon is 15.5 to 16 mm; and the diameter of the insulator is 10.3 to 10.7 mm. Therefore, the difference between the diameter of the insulator and the distance between opposite sides of the hexagon is 4.8 to 5.7 mm. By contrast, in the case of a pressure sensor-incorporating spark plug whose insulator is formed to have a diameter of 8.8 to 9.2 mm, the difference between the diameter of the insulator and the distance between opposite sides of the hexagon can be increased to 6.3 to 7.2 mm. Therefore, the end surface of the tightening portion on the side of the electricity reception terminal increases, so that the region available for disposing the output cable can be widened.
In the pressure sensor-incorporating spark plug of the sixth aspect of the invention, since the area of the end surface of the tightening portion on the side of the electricity reception terminal can be increased employing a thinner insulator, an output cable having a thick core wire can be disposed. Therefore, the hexagonal shape of the tightening portion can be maintained, to thereby enable use of an ordinary plug wrench. In addition, this arrangement can prevent the electrode terminal provided at the end portion of the output cable from coming off, and line loss can be reduced.